1. Technical Field
The present invention relates to a toner for developing an electrostatic image, a method for producing the toner and an image forming method.
2. Description of Related Art
In recent years, with the requirement of high image quality, a technology for forming a toner particle having a small particle size is making progress. However, it has been found that when the particle size is made small, resolution or sharpness of a full color image is surely satisfied to some degree but a variety of influences are exerted accompanying the minimization of the particle size. In addition, there arises a problem that as the toner particle is made smaller, charging property is more easily affected due to uneven distribution of colorants constructing the toner.
As a conventionally known toner with a small particle size, U.S. Pat. No. 6,296,980 (Japanese Patent Application Publication-Tokukai-2000-214629) discloses that a polymerized toner produced by a suspension polymerization or an emulsion polymerization can be controlled in the toner particle size or shape in a polymerization process in an aqueous medium, so that a toner having a small particle size, a unified particle size distribution and a round shape with no corners on the particle can be obtained. The polymerized toner has been noticed as a toner which makes it possible to reproduce a small dot image for a digital image due to the thin line reproducibility and high resolution thereof.
However, the polymerized toner tends to be inferior to a pulverized toner in dispersibility of colorants added in the toner. For example, in the case of producing the polymerized toner using the suspension polymerization, pigments as a colorant are dispersed in monomers and then the resultant dispersion is polymerized, however, there is a problem in that since viscosity of monomer droplets is easily increased with progress of the polymerization, coagulation of the colorant easily occurs. Further, in the case of producing the polymerized toner using the emulsion polymerization, there is a problem in that coagulation of the colorant is accelerated due to pH in an association process, namely, in a coagulation process, therefore, coagulation of the colorant easily occurs.
As described above, the polymerized toner has a problem that in the production process of the toner, coagulation of the colorant is liable to occur, as a result, dispersibility is easily deteriorated.
Particularly, in a multicolor image forming process, transparency is required in order to superimpose a plurality of color toners to form color images. In particular, when forming images on an OHP film, adequate transparency becomes a serious problem.
Further, when dispersibility of the colorant is poor, odor emitted from the toner as a final product is affected in some cases and therefore, improvement in dispersibility of the colorant is desired also from this point of view.
Further, after suspending an image forming apparatus under a high temperature and high humidity environment for a long period of time, there is caused a mixed state of a toner changed in the charge amount due to moisture absorption and a new toner, as a result, irregular density is generated on a half tone image having a halftone dot. In addition thereto, in the multicolor image forming process, a colorant added in the toner of developers of each color exerts an influence, as a result, a difference is caused in developing properties between the developers of each color, which incurs widening of color difference.
At present, in view of the above-described problems, there has been demanded a toner having more excellent dispersibility of a colorant in the toner and exhibiting more balanced hue than a conventional color toner.
In particular, as a magenta colorant, conventionally known are various ones such as azo lake pigments, anthraquinone dyes, quinacridone pigments, rhodamine dyes and lake pigments of these. Among these, there is desired a colorant which exhibits good chromaticity and has high heat resistance, low fog, good transmittance in OHP, and no odor when incorporated into the toner.
As to a technique for utilizing a conventionally known magenta colorant, for example, U.S. Pat. No. 6,455,215 (Japanese Patent Application Publication-Tokukaihei-9-179348) discloses a technique for utilizing as a colorant a magenta toner comprising C.I. Pigment Red 122, C.I. Pigment Red 57:1 or C.I. Pigment Red 81 (rhodamine dye) in which the magenta toner is used at a specific ratio in a binder resin, whereby light resistance or copy storage ability as a defect of rhodamine dyes is enhanced and narrow color reproduction area as a defect of azo lake pigments is improved.
Further, Japanese Patent Application Publication-Tokukaihei-10-312088 discloses a full-color image forming toner in which C.I. Pigment Red 122 and C.I. Pigment Red 57:1 as a magenta toner, C.I. Pigment Blue 15:3 as a cyan toner and C.I. Pigment Yellow 180 as a yellow toner each is used as a colorant, whereby color reproduction area is sufficiently improved and color image storage ability is improved.
However, it is still the case that even using the above-described magenta colorants, there is not obtained a toner which exhibits good chromaticity and has high heat resistance, low fog, good transmittance in OHP, and no odor when incorporated into the polymerized toner.